1. Field of the Invention
This invention relates to an AC to DC converter of an electrical power system.
2. Description of Related Art
A conventional power system design includes a Transformer Rectifier Unit (xe2x80x9cTRUxe2x80x9d) as a main DC power source, the output voltage of which varies with applied load. Due to the varying nature of its output voltage, the TRU has been considered unsuitable for charging batteries that are relied on for backup/auxiliary power supply. Specifically, the varying nature of the TRU output voltage does not enable a constant potential or constant current charging mode. To address this drawback, some systems use an ON/OFF device to connect and disconnect the battery from the TRU bus to improve charging performance.
Another conventional approach utilizes a DC generator as a main power source to generate a constant output voltage. Although such a DC generator is suitable for charging batteries at constant potential, this approach can neither limit the current flowing into the battery at the beginning of the starting cycle nor compensate for temperature variations, which strongly influence the battery State of Charge (xe2x80x9cSOCxe2x80x9d). Also, by not controlling the amount of trickle charge, the DC generator causes the battery to lose electrolytes over time.
Yet another conventional approach utilizes a Regulated Transformer Rectifier Unit (xe2x80x9cRTRUxe2x80x9d) as a main DC source with constant output voltage. The RTRU can charge batteries at constant potential, but can neither limit the current flowing into the battery nor compensate for temperature variations. Also, by not controlling the amount of trickle charge, the RTRU causes the battery to lose electrolytes over time.
Due to the above drawbacks, additional equipment, such as a separate battery charger, is commonly used in addition to the main DC power source. Since the charging voltage may be high at times, the battery must be disconnected from the main DC bus of the power system during charging. This requires special power bus arrangements and restricts the use of the battery as a bus presence, thereby affecting DC power system flexibility and quality.
The present invention addresses these drawbacks of conventional power system designs by providing a multi-functional AC to DC converter that controls AC to DC conversion as a function of at least one characteristic of a battery connected to a DC power bus, such that the voltage output by the converter to the DC power bus is varied to efficiently charge the battery while providing power for the bus connected loads. In one implementation of the present invention, a multi-functional AC to DC converter also adjusts the voltage output to the DC power bus based on a starting condition of an auxiliary power unit (xe2x80x9cAPUxe2x80x9d) so that the supplied voltage is suitable for starting the APU. The present invention is suitable for implementation in an aircraft power system to maintain the voltage output within the compliance range of the DC power bus (e.g., between 27.5 and 29.5 Vdc) while adjusting the output voltage to efficiently charge a battery connected to the bus and/or start an APU connected to the bus.
In one embodiment of the present invention, a multifunctional AC to DC converter includes a power control unit that monitors the current level drawn by a recharging battery and controls the voltage output by the AC to DC conversion circuitry based on the charging state of the battery. In a first power control stage, in which the battery is completely or nearly discharged, the power control unit controls the output voltage to a relatively low level (within the compliance range of the power system) to prevent the battery from drawing an uncontrolled or excessive amount of current. As the battery charges, and its internal impedance increases, the power control unit allows the output voltage to increase, thereby maintaining the current flowing to the battery at an adequate level for efficiently charging the battery. When the output voltage reaches a threshold level, the power control unit initiates a second control stage to maintain the output voltage constant at the elevated level (e.g., near the upper compliance limit of the DC power bus) so that the battery continues to charge, albeit at continually decreasing current. During this second power control stage, the power control unit in one implementation of the present invention controls output voltage as a function of battery temperature so that the output voltage is lower for higher battery temperatures and higher for lower battery temperatures. This temperature compensation improves the efficiency of the battery charging process. When the current drawn by the battery drops below a threshold level (e.g., indicating that the battery is about 80% percent recharged), the power control unit initiates a third power control stage to lower the output voltage to a nominal regulated level (e.g., 28 Vdc in an aircraft DC bus implementation) to trickle charge the battery (i.e., achieve constant potential trickle charge mode). This third power control stage prevents electrolyte loss during trickle charge.
By incorporating multiple functions in a single unit, the AC to DC converter of the present invention improves reliability because additional Line Replaceable Units (xe2x80x9cLRUsxe2x80x9d) and associated switching devices are not required to connect/disconnect the charging battery to/from the DC power bus and requires less control and power wiring. Furthermore, because the recharging battery may be continuously connected to the DC power bus, the present invention enables No-Break-Power-Transfers (NBPT) in the DC subsystem during AC power interrupts or other system failures and reduces DC system weight and cost as compared to a system requiring a separate battery charger.